Polyester-polyurethane hybrid resins are well-known in the art of thermoset molding compositions. The term hybrid describes a single, new type of polymer that is formed by the incorporation of the chemical groups and the properties of two different polymers, namely polyurethanes and unsaturated polyesters. Hybrid resins build molecular weight and toughness as they cure through the urethane chain-extension reaction, which occurs between the hydroxyl end groups on the polyester polyol and the isocyanate groups. Crosslinking occurs between the unsaturation in the polyester backbone and the styrene monomer, adding stiffness and thermal resistance. Thus, a unique blend of properties is obtained that cannot be acheived with either type of polymer alone. The hybrid resins are normally tougher than non-hybrid polyesters and stronger, stiffer and less expensive than polyurethanes. Polyester-polyurethane hybrid resins typically comprise a hydroxyl-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer, such as styrene, and a polyisocyanate. Polyester-polyurethane hybrid resins can be easily adapted to many common thermoset molding techniques employed in both the polyurethane and unsaturated polyester industries. Such hybrid resins are generally supplied as a two component system having an A-side and a B-side. The A-side typically contains the polyisocyanate and a free radical initiator, while the B-side typically contains the hydroxyl-terminated unsaturated polyester polyol/styrene solution and, optionally, fillers and/or additives.
One key problem that occurs during the hybrid cure is resin shrinkage which results in dimensional stability problems, such as warpage. Unacceptable surface appearances such as waviness or roughness result when hybrids are reinforced with fibers, such as glass. The resin shrinks around the glass fibers, allowing the fibers to show through to the surface of the molded article. This phenomenon is commonly termed glass print-through. It is desirable to reduce the shrinkage and improve the surface appearance (profile) of molded articles manufactured from hybrid resins.
U.S. Pat. No. 4,822,849, teaches reducing the shrinkage of hybrid resins by reducing both the styrene level and unsaturation level with the hybrid. Lower shrinkage is achieved by reducing the crosslink density, but this may lead to reduced thermal properties of the hybrid resin. U.S. Pat. No. 4,280,979, describes the preparation of unsaturated polyester polyols, which can be reacted with a polyisocyanate and a polymerizable ethylenically unsaturated monomer to produce polyurethane/vinyl copolymers. Both patents are incorporated herein by reference.
Low profile additives (LPA's) are added to unsaturated polyester resins to control shrinkage and improve dimensional stability and surface smoothness (profile). The LPA tends to phase separate from the polyester during cure, resulting in thermoplastic domains that induce stresses within the system. These stresses lead to the formation of internal imperfections, such as microcracks and microvoids, in molded products containing the LPA. The internal imperfections are beneficial because they reduce the amount of shrinkage that occurs during cure. Typical LPA-modified polyesters may contain up to 60 weight percent styrene monomer, and typically have high levels of unsaturation (greater than 5.5 moles unsaturation per kilogram of polyol). When there is more than one mole of styrene per equivalent of unsaturation in the polyester, the product tends to have high shrinkages; however, this shrinkage is counterbalanced by the micro-imperfections developed during the phase separation of the thermoplastic from the thermoset.
Severe glass print-through occurs in the typical conventional composite hybrid molding compositions. The failure of conventional LPA technology in commercial hybrid resins has been attributed to several factors. First, low levels of unsaturation in the polyester contribute to slow reactivity of the hybrid system: fast reactivity is considered to be one of the keys to achieving effective low profiling behavior. Also, the polyurethane reaction is considered to be slower than the unsaturated polyester crosslinking reaction; consequently, hybrid reactivity was always assummed to be significantly less than that for the corresponding polyester system. Second, the hybrid resin has a high matrix toughness compared to the unsaturated (non-hybrid) polyester resins because of the polyurethane component; therefore, the hybrid resin will not form microcracks or craze as easily. This cracking is essential for reducing shrinkage in a low profile system. And third, the low profile additive is highly soluble in the isocyanate component and, thus, is less likely to phase separate and low profile during the cure. To date, there have been no reports of observing effective low profiling behavior in hybrid resins.
Typical commercial hybrids possess low levels of unsaturation in the polyester (less than 4.0 moles/Kg) which typically results in slow reactivity. High levels of unsaturation and fast reactivity are generally believed to be necessary for achieving effective phase separation of the low profile additive. The unsaturation level in a polyol can be increased by substituting an unsaturated anhydride or acid for the saturated anhydride or acid in the polyol preparation. The reactivity of the polyol can be further increased by increasing the concentration of the fumarate (trans) isomer of unsaturation compared to the maleate (cis) isomer. This can be accomplished during the polyol preparation by catalyzing the isomerization with morpholine, starting with fumaric acid, or altering the addition procedures to take advantage of the equilibrium favorability of the fumarate isomer. However, it has been found that not all polyols with high levels of fumarate unsaturation are soluble at the styrene levels typically used in hybrid resins.
Although high unsaturation generally leads to faster reactivity, it can have a detrimental effect on physical properties, resulting in hybrids with reduced impact resistance and lower elongation properties. High unsaturation also leads to higher shrinkage during cure. The polyol compositions of the present invention have high fumarate levels but reduced levels of unsaturation, leading to good styrene solubility and improved hybrid physical properties. Unsaturation is reduced through the incorporation of saturated dicarboxylic acids into the polyol backbone. These systems unexpectedly exhibit effective low profile behavior, that was previously thought to be impossible at these low unsaturation levels since the reactivity would be too slow to induce phase separation of the low profile additive. However, the high fumarate content at the lower unsaturation level results in a polymer with sufficiently high reactivity to induce low profile behavior.